This invention concerns supported nanoporous carbon membranes with additives such as, but not limited to, titanium dioxide, small pore high silica zeolites, and poly(ethylene glycol). Such supported membranes are synthesized by pyrolysis of one or more layers of polymer containing materials, wherein at least one of the pyrolyzed layers is a polymer and additive mixture, on a porous substrate so as to produce a thin mixed matrix film with pores for the separation of small molecules.
Ceramic membranes can be used at high temperatures and in harsh environments as described in K. Keizer, H. Verweij, Progress in Inorganic Membranes, Chemtec, 16-1 (1996) 37. This kind of membrane is useful for catalysis as well as for separations.
Nanoporous carbons (NPC) or carbon molecular sieves (CMS) are good candidate materials for selective membrane formation. These can be prepared by the controlled pyrolysis of polymers such as poly(vinyl chloride) (PVC), poly(acrylonitrile) (PAN) and poly(furfuryl) alcohol (PFA), to name a few (See for example, H. C. Foley, Carbogenic Molecular Sieves, Microporous Mats., 4 (1995), 407, H. C. Foley, M. S. Kane, J. F. Goellner, in T. J. Pinnavaia and M. F. Thorpe, Access in Nanoporous Material, Plenum, New York, N.Y., 1995). Although amorphous, they are endowed with a regular nanostructure that leads to a pore network with narrowly distributed pore dimensions between 0.3 and 0.6 nm (R. K. Mariwala, H. C. Foley, Evaluation of Ultramicroporous Adsorptive Structure in poly(furfuryl alcohol) derived carbogenic molecular sieves, Ind. Eng. Chem. Res., 33 (1994) 607, M. S. Kane, J. F. Goellner, H. C. Foley, R. DiFrancesco, S. J. Billinge, L. F. Allard,: Symmetry breaking in nanostructure development of carbogenic molecular sieves: effect of morphological pattern formation on oxygen and nitrogen transport, Chem. of Mat., 8 (1996) 2159, V. Petkov, R. G. DiFrancesco, S. J. L. Billinge, M. Acharya, H. C. Foley, Simulation of Nanoporous Carbons: A Chemically Constrained Structure, Philos, Mag., B., 79 (1999) 1519, M. Acharya, M. S. Strano, J. P. Mathews, S. J. Billinge, V. Petkov, S. Subramoney and H. C. Foley, Simulation of Nanoporous Carbons: A Chemically Constrained Structure, Philos. Mag., B, 79 (1999) 1499). These materials can be grown on porous stainless steel support media by brush-coating, by spray-coating as has been reported (M. Acharya, B. A. Raich, H. C. Foley, M. P. Harold, J. J. Leroous, Metal-Supported Carbogenic Molecular Sieve Membranes: Synthesis and Applications, Inc. Eng. Chem., 8 (1997) 1924, M. Acharya, H. C. Foley, Spray-coating of nanoporous carbon membranes for air separation, J. Memb. Sci., 161 (1999) 1) or, preferably, by ultrasonic deposition of the polymer resin (M. B. Shiflett, H. C. Foley, Ultrasonic Deposition of High Selectivity Nanoporous Carbon Membrans, Science, 285 (1999) 1902).
Applicants describe alternative methods for preparing carbon molecular sieve layers on a stainless steel support. A protocol for increasing flux was followed by including additives such as inorganic oxides like titanium dioxide (TiO2), small pore high silica zeolite like SSZ-13, and polyethylene glycol (PEG) in the carbon film. Also, a protocol of pre-pyrolysis of the initial coatings in an inert environment at 800 K followed by heat treatment at lower temperatures 723 K and 650 K led to membranes with higher fluxes than previously reported. Initial coatings were also heat treated at lower temperature 423 K followed by heat treatment at 723 K after the final coatings were deposited as a means to more rapid membrane manufacturing. Furthermore, pyrolysis in the presence of a hydrogen atmosphere, also provides for high small molecule fluxes.
The present invention is a process for making thin film supported nanoporous carbon membranes with certain additive, such as titanium dioxide, small pore high silica zeolites, and poly(ethylene glycol).
Specifically, the invention includes a process for making a supported thin film nanopourous carbon membrane, having additive intermixed therein, comprising:
(a) coating a porous substrate with one or more layers of polymer intermixed with additive or a polymer alone, wherein at least one of the layers consists of polymer intermixed with additive, wherein the polymer or polymer intermixed with additive mixture is optionally dissolved in a solvent;
(b) drying the polymer coating by evaporating any solvent that may be present;
(c) then pyrolyzing the polymer or polymer mixture on the porous substrate so as to form a thin mixed matrix carbon film with pores for separation of small molecules; and, optionally,
(d) repeating steps (a), (b) and (c) one or more times.
The invention further concerns a polymer membrane supported on a porous substrate produced by the above process.
Preferred polymers for use in the process are poly(furfuryl) alcohol, poly(vinyl chloride) and poly(acrylonitrile).
Preferred additives are titanium dioxide, small pore high silica zeolites, and/or poly(ethylene glycol).
The initial polymer or polymer with additive, to be coated on the substrate, are generally dissolved in a solvent. Solvents that can be useful are selected from the group consisting of acetone, methylethylketone, benzene and toluene
Supports for the compositions described above include porous metals, porous ceramics, porous glasses and composites. Composites described herein are usefull as they can be fabricated into a number of shapes and of varying porosity for separation or catalysis uses.